Wireless communication systems are widely known in which subscriber stations communicate wirelessly in cells provided by base stations. Next generation wireless communications systems such as the Universal Mobile Telecommunications System (UMTS) and the UMTS Long Term Evolution (LTE) aim to offer improved services to the user compared to the existing systems. These systems are expected to offer high data rate services for the processing and transmission of a wide range of information, such as voice, video and IP multimedia data.
The subsequent description will refer to LTE by way of example, and the basic system architecture in LTE is illustrated in FIG. 1. In LTE the subscriber stations are referred to as user equipments (UEs) and the base stations are called enhanced NodeBs (eNBs). As can be seen, each UE 12 connects over a wireless link via a Uu interface to an eNB 11, which defines one or a number of cells for wireless communication. There is a network of eNBs referred to as the “eUTRAN”.
Each eNB 11 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities, including a Serving Gateway (S-GW 22), and a Mobility Management Entity (MME 21) for managing the system and sending control signalling to other nodes, particularly eNBs, in the network. In addition, a not-shown PDN [Packet Data Network] Gateway (P-GW) is present, separately or combined with the S-GW 22, to exchange data packets with any packet data network including the Internet. The wired links to the core network EPC (where EPC stands for Evolved Packet Core) are referred to as “backhaul” and employ Internet Protocol (IP).
As is implied by the above distinction between MME/S-GW and PDN-GW, in LTE, control signalling is separated from user data traffic; thus there is a “user plane” which is distinct from the “control plane”. FIG. 1, the S1 interface is labelled S1-U, the suffix -U denoting the user plane employed by the eNBs 11 for communicating user data to and from the S-GW 22. The S-GW is responsible for packet forwarding of user data on the downlink to the UE 12 and on the uplink. The S-GW 22 provides a “mobility anchor” for the user plane during handovers of a UE 12 from one eNB 11 to another.
In parallel to this, there is an interface S1-MME (sometimes called S1-C) via which the eNBs 11 exchange control messages with the MME 21. The main function of the MME 21, as its name suggests, is to manage mobility of the UEs 12, and it is a signalling-only entity; in other words, user data packets do not pass through the MME. The MME 21 is also responsible for controlling security (including authenticating users), and for EPS bearer control (see below). In practice, there may be several MMES forming a MME “pool”. One eNB can have several S1-MME interfaces towards several MMES.
In addition, as shown in FIG. 1, the eNBs 11 communicate among themselves by a (usually) wireless link, using an interface called X2 for mutual co-ordination, for example when handing over a UE 12 from one cell to another. There is only one X2 interface between two eNBs.
In the above configuration, communications among eNBs can be regarded as communications among peers (network nodes at the same hierarchical level) with the MME constituting a higher level entity in the system.
To support high data rates in next-generation wireless communication systems, relay nodes may be employed as capacity boosters between the subscriber stations and the base stations, or in other words (in the case of LTE) between UEs and eNBs. So-called mobile relay base stations are one possible type of relay node. Mobile relay base stations provide the same functionality as conventional base stations, but their link to the network is provided by using a similar radio interface as that used by UEs. In other words a mobile relay base station connects to an eNB (called a “donor” eNB or DeNB in LTE) in a similar way as an ordinary UE.
As the name suggests, a mobile relay base station is expected to have full base station functionality, in particular the ability to handle both user plane and control plane traffic. As the name also suggests, a mobile relay base station is mobile, in other words it can be assumed to move with a certain speed relative to the eNBs (which are generally fixed) and possibly also with respect to at least some UEs to which it is connected. Consequently, as the mobile relay base station moves it has to be handed over from a serving DeNB to another DeNB in order to maintain a continuous connection to the network. Since the mobile relay base station is expected to handle both control plane and user plane traffic, handover of a mobile relay base station conventionally involves handover of both types of traffic.
The basic LTE system architecture with a mobile relay base station is shown in FIG. 2. A UE 12 is connected to a mobile relay base station (labelled Mobile Relay 14 in FIG. 2) by a wireless link using the Uu radio interface. The Mobile Relay 14, in turn, connects to a DeNB 13 over a wireless link via the Un interface. The Donor (also called Anchor) eNB 13 may serve one or more mobile relays 14 (as well as possibly relay nodes of other kinds) and may also communicate directly with other UEs.
The User Plane data for UE 12 (user data traffic) is routed to the S-GW (labelled Serving Gateway 22). Typically the S-GW is used for several eNBs which may be interconnected by the X2 interface, which may be a real physical connection between the eNBs, or implemented as a logical connection via other network nodes. The DeNB 13 is the eNB that is connected to the Mobile Relay 14 using the radio interface (Un) and which uses similar radio resources to the Uu radio interface.
Although the Mobile Relay is treated like an eNB to some extent, and thus needs to send and receive S1-AP and/or X2-AP signalling, as is clear from FIG. 2 the S1 (and possibly X2) interface is carried between the RN and its DeNB over the Un interface.
Transmission of messages between nodes in a radio network, such as between mobile relays and eNBs, involves the use of multi-layer protocol stacks. On the transmission side, starting from the top of the stack at an application layer, each layer in the protocol stack processes units of data (packets) in some way, usually adding a header to the data unit before passing it down to the next lower layer or sub-layer. The headers include fields identifying the operations performed at that protocol layer. On the reception side, each layer decodes the header inserted in the corresponding transmission-side layer to allow reconstruction of a data unit, which is then passed up to the next higher layer.
FIG. 3 shows the protocol stacks in LTE for (1) User Plane and (2) Control Plane.
In the User Plane, user data traffic is transported via the two radio interfaces (Uu and Un). The User-Plane consists of Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and PHYsical (PHY) protocol layers. At the PDCP protocol layer, one protocol of particular relevance for present purposes is GTP-U, which is used on the S1 interface between the eNB and S-GW and on the S5/S8 interface between S-GW and P-GW. GTP stands for GPRS Tunneling Protocol and allows user data packets to be conveyed (“tunnelled”) between the P-GW and eNB.
The concept of “bearers” is important for achieving quality-of-service (QoS) in a packet-based network such as LTE. In general, a “bearer” can be thought of as an information transmission path of defined capacity, delay and bit error rate, etc. so as to enable a given service or control function to be provided. Various types or levels of bearer can be established, the radio part being set up using radio resource control or RRC. A single UE may have associated with it multiple bearers simultaneously for providing different services to the user.
FIG. 4 shows an EPS Bearer Service Architecture employed in LTE. The left side of the Figure represents the eUTRAN with the EPC occupying the middle part of the Figure. At the right-hand side, outside the LTE system as such, there is the Internet. The vertical bars represent the main entities in the user plane, from the UE 12 to eNB 11 through to S-GW 22 and P-GW 23, terminating in a peer entity 24 (such as an Internet web server) connected to the P-GW 23. Communication takes place between the S-GW and P-GW over an S5 or S8 interface. To provide an end-to-end service between the UE 12 and Peer Entity 24 (as indicated by the upper horizontal band in the figure), the system sets up “bearers” as shown. An EPS Bearer represents the entire connection within the LTE system; it constitutes a QoS flow for a particular service. The connection continues outside the LTE system via an External Bearer.
The EPS Bearer is made up, in turn, of a radio bearer over the link between the UE 12 and eNB 11, and an S1 Bearer between the eNB 11 and S-GW 22. A further Bearer (S5/S8 Bearer) is set up between the S-GW 22 and P-GW 23. Each Bearer can be regarded as a “tunnel” in a given protocol layer for transport of packets, connecting the end points for the duration of a particular service or “session”, e.g. voice call or download. Thus, the radio bearer transports the packets of the higher-layer EPS Bearer between the UE 12 and eNB 11, and the S1 Bearer transports the packets of the EPS Bearer between the eNB 11 and S-GW 22. Bearer control through RRC, mentioned previously, includes the setting up of bearers for a particular session so as to ensure sufficient QoS, taking into account the resource situation in the E-UTRAN and existing sessions already in progress. It also involves the modification and release of radio bearers.
Further details of handover and signalling procedures in LTE are contained in the following documents which are hereby incorporated by reference:
3GPP TS36.300 “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2”
3GPP TS36.331 “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”
3GPP TS36.413 “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1-AP)”
3GPP TS36.423 “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2-AP)”
As already mentioned, in a wireless communication system employing mobile relays as in FIG. 2, there is a requirement for handover of the mobile relays and correspondingly of the UEs connected to those mobile relays. Mobility management functions in LTE and UMTS networks for UEs in connected mode handle all necessary steps for handover. These steps include processes that precede the final HO handover decision on the source network side (control and evaluation of UE and base station measurements), preparation of resources on the target network side, commanding the UE to the new radio resources and finally releasing resources on the source network side. Handover of UEs involves transferring, from one eNB to another, all the information related to the UE, called its “context”. Mobility management also contains mechanisms to transfer context data between the eNBs, and to update node relations on both control plane and user plane.
A typical handover procedure in LTE networks, taken from the above mentioned 3GPP TS36.300, is illustrated in FIG. 5, showing three phases of handover respectively labelled “Handover Preparation”, “Handover Execution” and “Handover Completion”.
Suppose that a connected-mode UE 12 (or likewise a mobile relay base station acting as a UE towards its donor base station) is connected to a Source eNB 11 providing a serving cell, and can receive at least reference signals from a neighbour cell provided by a Target eNB 11. In a step 1. “Measurement Control”, the UE 12 is triggered to send measurement report by the rules set by i.e. system information, specification etc. (see 3GPP TS36.331). In a step 2. “Measurement Reports”, UE 12 performs measurements of attributes of the serving and neighbour cells. Step 3 “HO decision” is for Source eNB 11 to make a decision based on measurement report and RRM information to hand over the UE 12. Then, (4. “Handover Request”) the Source eNB issues a handover request to the Target eNB, passing necessary information to prepare the handover at the target side. In a step 5. “Admission Control”, admission control may be performed by the Target eNB to determine whether or not it agrees to accept the UE. Then (6. “Handover Request Ack.”) the Target eNB 11 prepares the HO and sends the handover request Ack. to the Source eNB, in which a handover command is included for the Source eNB to forward the command in the form of a message labelled “7. RRC Conn. Reconf.mobilityControlinfo”, to instruct the UE to connect to the target cell.
Several necessary steps are performed on the network side to ensure a lossless user plane path switch, in other words minimum interruption in the data packets being transmitted to or from the UE. These include a step 8. “SN Status Transfer” by which the Source eNB informs the Target eNB of the Sequence Number (SN) up to which it has successfully delivered data packets, in order for the Target eNB to know at which packet to start transmission.
After receiving the handover command, UE performs synchronisation to Target eNB (9. “Synchronization”) and accesses the target cell. The Target eNB responds (10. “UL Allocation+TA for UE”) with uplink allocation and timing advance. When the UE has successfully accessed the target cell, the UE sends a message (11. “RRC Connection Reconfiguration Complete”) to the Target eNB 11 to confirm the completion of handover.
The subsequent steps 12.-15. in FIG. 5 can be summarised as a user plane path switch, which changes the DL user plane data delivery path from the path: S-GW→Source eNB to: S-GW→Target eNB. Finally, the MME 21 confirms (16. Path Switch Req. Ack) the handover and the Target eNB 11 then sends a message (17. “UE Context Release”) to instruct the Source eNB to release the resources previously allocated to the handed-over UE.
As will be understood, the above handover is a handover of both the signalling and data traffic, or in other words both the control plane and user plane. This is what is normally understood by “handover” in a wireless communication system.
The problem addressed are that in systems such as LTE and UMTS, mobile relay base stations are deployed in certain areas, for example, peak hours in city centres. Mobile relays installed in vehicles (such as, cars, buses) are provided primarily for the use of UEs of passengers of those vehicles. However, such mobile relays move with relatively low speed, which also allows other users such as pedestrians (outside of the vehicles) with low speed to access these mobile relays. Thus, in this scenario, mobile relays are deployed as capacity boosters serving UEs which have a low relative speed relative to the mobile relays. The key issue in this scenario is how to support handovers of the UEs which are necessitated by the movement either of mobile relays or of UEs.